Polymer microparticles having a high specific surface area and a spherical shape have been used as additives for molding various polymers, and modifying/improving materials. Specific applications include coating molding composed of formation of a coating film of polymer microparticles; use for various moldings of films, sheets and the like; and use of modifying agents of cosmetics, additives for toner, additives for paints, additives for molded products, light diffusing agents of film and the like.
There has been known a technique in which, in coating molding and various moldings by polymer microparticles, polymer microparticles are arranged or a powder layer is formed, and then polymer microparticles are fused with each other by applying thermal energy to thereby mold the polymer into a desired form. In those applications, fusion of particles is promoted if polymer microparticles have a large specific surface area, thus enabling coating and molding at a lower temperature in a shorter time.
The technique to increase the specific surface area of polymer microparticles includes a technique of making particles porous. In porous polymer microparticles in which a surface of particles is in a porous form, thermal energy applied to a surface of polymer microparticle per unit time increases when thermal energy is applied to polymer microparticles to make a molded body, thus making it possible to obtain molded objects at smaller energy in a shorter time.
The technique to increase a specific surface area of polymer microparticles also includes a technique of decreasing a particle diameter of particles, and a technique of deforming particles. However, the technique of decreasing a particle diameter of particles is not practically preferable since it may cause deterioration of powder handleability and exert an adverse influence on the work environment. The technique of deforming particles is not practically preferable since it may cause unevenness in a molten state due to non-uniform application manner of thermal energy.
Polymer microparticles are added to paint, and also used as additives to change the appearance and texture of paint. For example, polymer microparticles are added as a matting agent of paint.
In that case, light scattering properties of polymer microparticles are utilized. Particles having high shading effect, which is capable of scattering light in multiple directions, imparts high matte effect of paint. The form suitable for imparting high matte effect of polymer microparticles is preferably a porous shape.
Inorganic particles such as silica particles are known as additives for paint. Inorganic particles are not practically preferable since inorganic particles are likely to undergo gravity settling as compared with polymer microparticles, leading to separation from paint.
A polyphenylene sulfide (hereinafter sometimes abbreviated to PPS) resin has properties suitable for use as engineering plastics such as excellent heat resistance, chemical resistance, solvent resistance, and electrical insulation properties, and is therefore used mainly in injection molding applications and extrusion molding applications, for example, various electric components, mechanical components, and automobile components; and additives for modifying agents such as oil and grease of various sliding portions.
There is a high demand in the fact that such excellent PPS resin is formed into microparticles in a porous state and the thus obtained microparticles are used for various moldings, or used as coating agents, heat-resistant additives, and modifying agents or additives of paint. However, it is significantly difficult to form the PPS resin into porous microparticles because of the below-mentioned technical restriction.
Some techniques mentioned below have been proposed as a method of obtaining PPS microparticles. In Japanese Unexamined Patent Publication (Kokai) No. 10-273594, PPS and a thermoplastic polymer other than that are melt-kneaded to form a resin composition having a sea-island structure composed of PPS as an island component and the other thermoplastic polymer as a sea component, and then the sea component is dissolved and washed to obtain spherical PPS resin microparticles. Otherwise, a method of producing PPS microparticles utilizing precipitation by cooling is known. In Japanese Unexamined Patent Publication (Kokai) No. 61-287927, a reaction vessel is cooled after polymerization of a PPS resin to take out PPS as a powder. In Japanese Unexamined Patent Publication (Kokai) No. 2010-106232, a solution prepared by dissolving a PPS resin is put in a heated/pressurized state, and the solution is jetted into a solvent through a nozzle and then quickly cooled to precipitate PPS microparticles.
However, we found that, according to the method disclosed in aforementioned JP '594, the PPS resin and the other thermoplastic resin are melt-kneaded to form the sea-island structure, thus forming islands serving as the origin of PPS microparticles so that only PPS microparticles having a smooth surface can be produced due to the operation of the surface tension. In the precipitation method disclosed in JP '927, PPS powders are fused with each other in the precipitation step and it is difficult to prevent deformation of the PPS powder and widening of the particle diameter distribution. Also in the method disclosed in JP '232, droplets formed by operation of the surface tension are cooled to precipitate in the jetting step so that only PPS microparticles having a smooth surface can be produced.
Meanwhile, porous PPS microparticles are expected to exhibit satisfactory operability in molding applications or to exhibit matte effect as paint additives. To meet the expectation, porous PPS microparticles, which are true sphere-like particle shape and have uniform particle diameter, are desired. However, known PPS microparticles did not satisfy those properties.
It could therefore be helpful to provide porous polyphenylene sulfide microparticles in a practically applicable level.